It is proposed to examine the chromosomes of a large population of mentally retarded individuals under the age of 18 together with those of a similar number of control individuals. A giemsa banding technique will be used and particular emphasis will be placed on the mutation rate of structural abnormalities of the chromosomes and on the contribution of this type of aberration to the etiology of physical abnormality and mental retardation. An investigation of cytogenetic polymorphisms utilizing the C, G, and Q banding techniques is also proposed. This would first compare the type and incidence of cytogenetic polymorphisms in a wide variety of different ethnic groups living in Hawaii and secondly, search for disease association with polymorphic features by comparing the type and incidence of the latter between the mentally and physically abnormal patients and controls.